Blast from a US Navy fuel air explosive used against a decommissioned ship, USS McNulty, 1972

A thermobaric weapon is a type of explosive that utilizes oxygen from the surrounding air to generate an intense, high-temperature explosion, and in practice the blast wave of such a weapon produces a typically significantly longer duration than a conventional condensed explosive. The fuel-air bomb is one of the most well-known types of thermobaric weapons.

Most conventional explosives consist of a fuel-oxidizer premix (gunpowder, for example, contains 25% fuel and 75% oxidizer), whereas thermobaric weapons are almost 100% fuel, so thermobaric weapons are significantly more energetic than conventional condensed explosives of equal weight. Their reliance on atmospheric oxygen makes them unsuitable for use underwater, at high altitude, and in adverse weather. They do, however, cause considerably more destruction when used inside confined environments such as tunnels, caves, and bunkers - partly due to the sustained blast wave, and partly by consuming the available oxygen inside those confined spaces.

There are many different types of thermobaric weapons rounds that can be fitted to hand-held launchers.[1]

In contrast to condensed explosive, where oxidation in a confined region produces a blast front from essentially a point source, a flame front accelerates to a large volume producing pressure fronts both within the mixture of fuel and oxidant and then in the surrounding air.[2]

Thermobaric explosives apply the principles underlying accidental unconfined vapor cloud explosions, which include those from dispersions of flammable dusts and droplets.[3] Previously, such explosions were most often encountered in flour mills and their storage containers, and later in coal mines; but, now, most commonly in discharged oil tankers and refineries, including an incident at Buncefield in the UK in 2005 where the blast wave woke people 150 kilometres (93 mi) from its centre.[4]

A typical weapon consists of a container packed with a fuel substance, in the center of which is a small conventional-explosive "scatter charge". Fuels are chosen on the basis of the exothermicity of their oxidation, ranging from powdered metals, such as aluminium or magnesium, to organic materials, possibly with a self-contained partial oxidant. The most recent development involves the use of nanofuels.[5][6]

A thermobaric bomb's effective yield requires the most appropriate combination of a number of factors; among these are how well the fuel is dispersed, how rapidly it mixes with the surrounding atmosphere, and the initiation of the igniter and its position relative to the container of fuel. In some designs, strong munitions cases allow the blast pressure to be contained long enough for the fuel to be heated up well above its auto-ignition temperature, so that once the container bursts the super-heated fuel will auto-ignite progressively as it comes into contact with atmospheric oxygen.[7][8][9][10][11][12][13][14][15][16][17]

Conventional upper and lower limits of flammability apply to such weapons. Close in, blast from the dispersal charge, compressing and heating the surrounding atmosphere, will have some influence on the lower limit. The upper limit has been demonstrated strongly to influence the ignition of fogs above pools of oil.[18] This weakness may be eliminated by designs where the fuel is preheated well above its ignition temperature, so that its cooling during its dispersion still results in a minimal ignition delay on mixing. The continual combustion of the outer layer of fuel molecules as they come into contact with the air, generates additional heat which maintains the temperature of the interior of the fireball, and thus sustains the detonation.[19][20][21]

In confinement, a series of reflective shock waves are generated,[22][23] which maintain the fireball and can extend its duration to between 10 and 50 ms as exothermic recombination reactions occur.[24] Further damage can result as the gases cool and pressure drops sharply, leading to a partial vacuum. This effect has given rise to the misnomer "vacuum bomb". Piston-type afterburning is also believed to occur in such structures, as flame-fronts accelerate through it.[25][26]

A fuel-air explosive (FAE) device consists of a container of fuel and two separate explosive charges. After the munition is dropped or fired, the first explosive charge bursts open the container at a predetermined height and disperses the fuel in a cloud that mixes with atmospheric oxygen (the size of the cloud varies with the size of the munition). The cloud of fuel flows around objects and into structures. The second charge then detonates the cloud, creating a massive blast wave. The blast wave destroys unreinforced buildings and equipment and kills and injures people. The antipersonnel effect of the blast wave is more severe in foxholes, on people with body armor, and in enclosed spaces such as caves, buildings, and bunkers.

Fuel-air explosives were first developed, and used in Vietnam, by the United States. Soviet scientists, however, quickly developed their own FAE weapons, which were reportedly used against China in the Sino-Soviet border conflict and in Afghanistan. Since then, research and development has continued and currently Russian forces field a wide array of third-generation FAE warheads.

The [blast] kill mechanism against living targets is unique–and unpleasant.... What kills is the pressure wave, and more importantly, the subsequent rarefaction [vacuum], which ruptures the lungs.... If the fuel deflagrates but does not detonate, victims will be severely burned and will probably also inhale the burning fuel. Since the most common FAE fuels, ethylene oxide and propylene oxide, are highly toxic, undetonated FAE should prove as lethal to personnel caught within the cloud as most chemical agents.

According to a U.S. Central Intelligence Agency study,[27] "the effect of an FAE explosion within confined spaces is immense. Those near the ignition point are obliterated. Those at the fringe are likely to suffer many internal, and thus invisible injuries, including burst eardrums and crushed inner ear organs, severe concussions, ruptured lungs and internal organs, and possibly blindness." Another Defense Intelligence Agency document speculates that because the "shock and pressure waves cause minimal damage to brain tissue…it is possible that victims of FAEs are not rendered unconscious by the blast, but instead suffer for several seconds or minutes while they suffocate."[28]

The Russian armed forces have developed thermobaric ammunition variants for several of their weapons, such as the TGB-7V thermobaric grenade with a lethality radius of 10 metres (33 ft), which can be launched from a RPG-7. The GM-94 is a 43 mm pump-action grenade launcher which is designed mainly to fire thermobaric grenades for close quarters combat. With the grenade weighing 250 grams (8.8 oz) and holding a 160 grams (5.6 oz) explosive mixture, its lethality radius is 3 metres (9.8 ft); however, due to the deliberate "fragmentation-free" design of the grenade, 4 metres (13 ft) is already considered a safe distance.[31] The RPO-A and upgraded RPO-M are infantry-portable RPGs designed to fire thermobaric rockets. The RPO-M, for instance, has a thermobaric warhead with a TNT equivalence of 5.5 kilograms (12 lb) of TNT and destructive capabilities similar to a 152 mm High explosive fragmentation artillery shell.[32][33] The RShG-1 and the RShG-2 are thermobaric variants of the RPG-27 and RPG-26 respectively. The RShG-1 is the more powerful variant, with its warhead having a 10 metres (33 ft) lethality radius and producing about the same effect as 6 kg (13 lb) of TNT.[34] The RMG is a further derivative of the RPG-26 that uses a tandem-charge warhead, whereby the precursor HEAT warhead blasts an opening for the main thermobaric charge to enter and detonate inside.[35] The RMG's precursor HEAT warhead can penetrate 300 mm of reinforced concrete or over 100 mm of rolled homogeneous armour, thus allowing the 105 millimetres (4.1 in) diameter thermobaric warhead to detonate inside.[36]

The other examples include the SACLOS or millimeter wave radar-guided thermobaric variants of the 9M123 Khrizantema, the 9M133F-1 thermobaric warhead variant of the 9M133 Kornet, and the 9M131F thermobaric warhead variant of the 9K115-2 Metis-M, all of which are anti-tank missiles. The Kornet has since been upgraded to the Kornet-EM, and its thermobaric variant has a maximum range of 10 kilometres (6.2 mi) and has the TNT equivalent of 7 kilograms (15 lb) of TNT.[37] The 300 mm 9M55S thermobaric cluster warhead rocket was built to be fired from the BM-30 SmerchMLRS. A dedicated carrier of thermobaric weapons is the purpose-built TOS-1, a 24-tube MLRS designed to fire 220 mm caliber thermobaric rockets. A full salvo from the TOS-1 will cover a rectangle 200x400 metres.[38] The Iskander-Mtheatre ballistic missile can also carry a 700 kilograms (1,500 lb) thermobaric warhead.[39]

Many Russian Air Force munitions also have thermobaric variants. The 80 mm S-8 rocket has the S-8DM and S-8DF thermobaric variants. The S-8's larger 122 mm brother, the S-13 rocket, has the S-13D and S-13DF thermobaric variants. The S-13DF's warhead weighs only 32 kg (71 lb) but its power is equivalent to 40 kg (88 lb) of TNT. The KAB-500-OD variant of the KAB-500KR has a 250 kg (550 lb) thermobaric warhead. The ODAB-500PM and ODAB-500PMV unguided bombs carry a 190 kg (420 lb) fuel-air explosive each. The KAB-1500S GLONASS/GPS guided 1,500 kg (3,300 lb) bomb also has a thermobaric variant. Its fireball will cover over a 150-metre (490 ft) radius and its lethality zone is a 500-metre (1,600 ft) radius.[40] The 9M120 Ataka-V and the 9K114 Shturm ATGMs both have thermobaric variants.

In September 2007 Russia exploded the largest thermobaric weapon ever made. The weapon's yield was reportedly greater than that of the smallest dial-a-yield nuclear weapons at their lowest settings.[41][42] Russia named this particular ordnance the "Father of All Bombs" in response to the United States developed "Massive Ordnance Air Blast" (MOAB) bomb whose backronym is the "Mother of All Bombs", and which previously held the accolade of the most powerful non-nuclear weapon in history.[43] The bomb contains an about 7 tons charge of a liquid fuel such as ethylene oxide, mixed with an energetic nanoparticle such as aluminium, surrounding a high explosive burster[44] that when detonated created an explosion equivalent to 44 metric tons of TNT.

The XM1060 40-mm grenade is a small-arms thermobaric device, which was delivered to U.S. forces in April 2003.[45] Since the 2003 Invasion of Iraq, the US Marine Corps has introduced a thermobaric 'Novel Explosive' (SMAW-NE) round for the Mk 153 SMAW rocket launcher. One team of Marines reported that they had destroyed a large one-story masonry type building with one round from 100 yards (91 m).[46]

The AGM-114N Hellfire II, first used by U.S. forces in 2003 in Iraq, uses a Metal Augmented Charge (MAC) warhead that contains a thermobaric explosive fill using aluminium powder coated or mixed with PTFE layered between the charge casing and a PBXN-112 explosive mixture. When the PBXN-112 detonates, the aluminium mixture is dispersed and rapidly burns. The resultant sustained high pressure is extremely effective against people and structures.[47]

US Navy BLU-118B being prepared for shipping for use in Afghanistan, 5 March 2002

The first experiments with thermobaric weapon were conducted in Germany during World War II and were led by Mario Zippermayr. The German bombs used coal dust as fuel and were extensively tested in 1943 and 1944, but did not reach mass production before the war ended.

Reports by the rebel fighters of the Free Syrian Army claim the Syrian Air Force used such weapons against residential area targets occupied by the rebel fighters, as for instance in the Battle for Aleppo[59] and also in Kafar Batna.[60] A United Nations panel of human rights investigators reported that the Syrian government used thermobaric bombs against the rebellious town of Qusayr in March 2013.[61]

The Shmel-M is a shoulder-launched thermobaric weapon. The TOS-1A is a multiple-rocket launcher mounted on a T-72 tank chassis. It can fire 24 220-millimeter rockets each packing a 100-kilogram warhead. The rockets' range is about six kilometers.

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